Electronic instruments for computing, test, and measurement often use small accessory modules to provide addition functional features. Such features are not usually cost effective to include in a standard instrument, but are nevertheless desired by a limited portion of the instrument's users. These modules are typically sold as an option at additional cost, and may be purchased and added by the user at a later date, as the need arises. In addition, such modules can be replaceable, so that a module providing one specialized function can be removed, and a substitute module providing a different capability can replace it as needed.
Accessory modules sometimes include stored data or programs, or sometimes include hardware or circuitry. Typically, such modules are installed in an enclosed compartment at the rear or side of the instrument, so that scarce area on the front panels of such instruments is not wasted, allowing front panel area to be used for interface devices, switches, displays, and other elements requiring access during normal use.
While the positioning of modules where they are not normally visible does not present an undue inconvenience for users who only rarely need to add or remove modules, it makes it difficult for a user to readily determine whether and which modules are installed. Some instruments provide module presence information to a user operating the unit and making an appropriate query. However, this is inconvenient or impractical in some circumstances. For instance, in a large organization having an inventory of numerous similar instruments, a manager of the instrument depot or a user seeking to borrow an instrument sometimes needs to determine which of several dozen similar instruments contains the modules with the special capabilities required. It would be impractical to power up and query each instrument, and external labeling systems can be unaesthetic, outdated, and inconvenient.